To enhance the efficiency of eavesdropping detection in the “Ping-pong” protocol, an improved “Pingpong” protocol based on four-qubit genuine entangled state is presented. The four-qubit genuine entangled state is used to detect eavesdroppers. In the security analysis, the method of the entropy theory is introduced, and three detection strategies are contrasted quantitatively by using the constraint between the information eavesdropper can obtain and the interference introduced. If the eavesdropper wants to obtain the same amount of information, she must face a larger detection probability in the proposed scheme than the other two. At last, the security of the proposed protocol is discussed. The analysis results indicate that the improved “Ping-pong” protocol in this paper is more secure than the other two.